Field
Aspects of the present invention generally relate to a technique for controlling a current supplied to a semiconductor laser provided in an image forming apparatus.
Description of the Related Art
An electrophotographic image forming apparatus forms an electrostatic latent image on a photosensitive member by exposing the photosensitive member to a laser beam output from a semiconductor laser. The electrostatic latent image formed on the photosensitive member is developed with a toner, and the developed toner image is transferred onto a recording medium. Then, the toner image transferred on the recording medium is fixed, and thus an image is formed on the recording medium.
The semiconductor laser emits a laser beam by receiving a driving current. The semiconductor laser has been known to have light emission delay characteristics. As illustrated in FIG. 8, the driving current ((b) in FIG. 8) is supplied to the semiconductor laser based on an image signal ((a) in FIG. 8). The light emission delay is a characteristic in which the rising of a light amount wave form of the laser beam lags a supply start timing of the driving current as illustrated in (c) in FIG. 8. Thus, there is a time lag between the supply of the driving current to the semiconductor laser and the output of the laser beam of a target light amount. Accordingly, with the electrophotographic image forming apparatus using the semiconductor laser as a light source for exposing the photosensitive member to light, the amount of light to which the photosensitive member is exposed might be insufficient due to the light emission delay characteristics of the semiconductor laser. As a result, an image with a density lower than a desired level might be output.
In view of such a problem, Japanese Patent Application Laid-Open No. 5-328071 discusses a method for preventing the output of an image with a low density due to an insufficient light amount at the time of the rising of the light amount. Specifically, a correction current that attenuates at a predetermined time constant from a peak value is generated by a differential circuit and, at the supply start timing of a driving current to a semiconductor laser, the correction current is superimposed on the driving current.
However, in an image forming apparatus in which the peak value of a correction current is set to a fixed value as discussed in Japanese Patent Application Laid-Open No. 5-328071, the light amount cannot be sufficiently corrected when the light amount of the laser beam to which the photosensitive member is exposed is adjusted based on the state of the image forming apparatus. For example, the following case is considered. Specifically, when the light amount of the laser beam to which the photosensitive member is exposed is adjusted to a first light amount, a driving current supplied to a semiconductor laser is adjusted to a first current value. Furthermore, when the light amount is adjusted to a second light amount smaller than the first light amount, a driving current supplied to the semiconductor laser is adjusted to a second current value smaller than the first current value. Here, the rate of the peak value of the correction current with respect to the first current value is different from the rate of the peak value of the correction current with respect to the second current value. Thus, when the peak value of the correction current is set with one of the first and the second current values used as a reference value, the rising of the light amount is not sufficiently corrected if the value of the driving current supplied to the semiconductor laser is set to the current value that is not used as the reference value.